1. Field of the Invention
The present invention relates to multifilamentary oxide superconducting wires and methods of producing the same and stranded oxide superconducting wires formed of the multifilamentary wires that are twined together and methods of producing the same, and particularly to multifilamentary oxide superconducting wires having a relatively high critical current density (Jc) while having a relatively low aspect ratio and methods of producing the same and stranded wires formed of such multifilamentary wires and methods of producing the same.
2. Description of the Related Art
A multifilamentary oxide superconducting wire produced by a xe2x80x9cpowder-in-tubexe2x80x9d process is expected to be applied to superconducting cables, superconducting equipment and the like with liquid-nitrogen cooling. In particular, a tape-shaped multifilamentary superconducting wire having a copper oxide covered with a stabilizing metal is overcoming challenges such as providing a viable critical current density, a viable length, a viable mass-production technique and the like. Typically, a tape-shaped multifilamentary wire is prepared in the powder-in-tube process by packing raw material powder for oxide superconductor in a silver pipe, drawing the same to prepare a monofilamentary wire, inserting a large number of monofilamentary wires into a silver pipe (to prepare a multifilamentary structure), drawing and rolling the multifilamentary structure, and heat-treating the multifilamentary structure to sinter it.
Because of the crystallographic property of the oxide superconductor material, it is advantageous for high Jc that the superconducting filament is shaped flat having a high aspect ratio. Such a flat filament is typically obtained by pressing or rolling. As such, a wire of high Jc is provided in the form of a tape having a relatively high aspect ratio. For example, a typical tape-shaped wire has an aspect ratio of approximately 20, having a width of approximately 4 mm and a thickness of approximately 0.2 mm.
A tape-shaped wire, however, shows anisotropic properties in the direction of the magnetic field applied thereto and readily causes an ac loss in a fluctuating magnetic field. If a cable conductor is formed of a set of the tape-shaped wires, unevenness in impedance between the wires can disadvantageously cause a current drift and the like so that the conductor can have a significant ac loss. In general, the problems of current drift and ac loss can rather be overcome by a stranded structure. It is difficult, however, to form a stranded wire by assembling a large number of tape-shaped wires.
In contrast, round wires having a round cross section are suitable for forming a stranded wire. For example, Japanese Patent Laying-Open No. 4-262308 discloses a round wire with metal, silver or silver alloy and a cylindrical oxide superconductor alternately, concentrically stacked, as seen in cross section. The publication discloses that a multiannular structure having the metal and the oxide superconductor alternately stacked is provide, so that the distance between the interfaces of the oxide superconductor and the metal can be reduced, more specifically, the adjacent interfaces can be not more than 100 xcexcm distant from each other, to provide a c-axis orientation. However, this wire""s Jc is a value smaller by one digit than the tape-shaped wire and is not a practically applicable level.
Japanese Patent Laying-Open No. 5-266726 discloses a method of producing an oxide superconducting wire round in cross section with a critical current density hardly depending on the direction of the magnetic field applied. More specifically, the method includes the step of packing a powdery oxide superconductor in a space formed between a metal tube and a metal core inserted therein, the step of performing plastic working on the tube, and the step of sintering the obtained wire. This method provides a superconducting wire having a structure with a cylindrical, oxide superconducting layer between the metal core and the metal tube. This method would not be suitable for producing a wire having a large number of superconducting filaments suitable for coils or the like.
Another example of attempting to enhance the critical current density of a round superconducting wire is disclosed in Cryogenics (1992) Vol. 32, No. 11, 940-948. As regards the round wire disclosed in this literature, 55 monofilamentary rods rectangular in cross section are arranged concentrically in three layers in a silver tube. In the report, the critical current of the obtained wire has not been measured. The wire disclosed in the literature, however, would not have a high Jc.
U.S. Pat. No. 5,347,085 discloses a multifilamentary oxide superconducting wire round in cross section, wherein a plurality of flat, oxide superconducting filaments are arranged in a stabilizer with their widths oriented radially. Thus, in the cross section of the wire, the filaments have their widths oriented circumferentially. In general, this wire would hardly provide higher Jc, because such a process of the wire that performs plastic working in the direction of the widths of the filaments would not enhance a c-axis orientation that can provide a high Jc.
U.S. Pat. No. 5,885,938 discloses a multifilamentary oxide superconducting wire having a low aspect ratio in cross section. This wire has an oxide superconducting crystal whose c-axis is oriented perpendicular to the longitudinal direction of the wire. In this wire, the filament has an aspect ratio typically equal to that of the wire. This low aspect ratio wire is obtained by performing plastic working on a multifilamentary precursor tape and heat-treating it. In this technique, the step of processing a high aspect ratio tape into a low aspect ratio wire would not provide a high Jc. Thus this technique would hardly provide a high Jc wire comparable to the tape-shaped wire.
Japanese Patent Laying-Open Nos. 9-259660 and 11-39963 disclose a multifilamentary oxide superconducting wire and a method of producing the same according to the powder-in-tube process. The improved method disclosed in the publications includes the steps of: packing an oxide superconductor or raw material powder therefor in a tube formed of a stabilizing material; performing plastic working on the tube with the powder packed therein to obtain a tape-shaped wire; packing a plurality of such tape-shaped wires in a tube formed of a stabilizing material; performing plastic working on the tube with the tape-shaped wires packed therein to obtain a wire having a substantially round cross section or a substantially equilaterally polygonal cross section having at least six angles; and heat-treating the wire to produce a sintered body of the oxide superconductor, wherein the powder portion of the tape-shaped wire to be packed in the tube is in the shape of a ribbon having an aspect ratio of 4 to 40 and the wire thermally treated has oxide superconducting filaments each having a thickness in a range from 5 xcexcm to 50 xcexcm. In this method, sufficiently compressed tape-shaped wires can be packed in a tube in an appropriate arrangement to enhance the crystal orientation of the oxide superconducting for a high Jc. However, a higher Jc is also desired over this technique.
Japanese Patent Laying-Open No. 6-68727 (filed on Aug. 19, 1992) discloses a production method including the steps of: providing an oxide superconducting film on an abraded, long substrate; and cutting the substrate to obtain a plurality of superconducting wires. According to this method, the substrate is cut so as to produce a superconducting wire having a lower aspect ratio than the substrate. In this method, however, it is difficult to obtain a multifilamentary wire.
Conventionally there is also a technique, as shown in FIG. 1, stacking and bonding a plurality of tape-shaped, oxide superconducting wires 1 together to provide a block 2. In each tape-shaped wire 1, oxide superconducting filaments are completely covered with a stabilizing material. Block 2 has a lower aspect ratio than tape-shaped wire 1. Block 2 thus obtained, however, has relatively large width and thickness so that it is bulky and unsuitable as a strand for use in producing a stranded wire. Furthermore, block 2 can contain an excessive amount of the stabilizer to provide a Jc per the cross section area of the wire (i.e., overall Jc) reduced.
D. C. Larbalestier et al., Physica C 221 (1994) 299-303 and G. Grasso et al., Physica C 241 (1995) 45-52 disclose that a relatively short, monofilamentary oxide superconducting tape is cut to provide a plurality of pieces thereof for the measurement of the Jc profile in the oxide super conductor. These reports show an interesting result with respect to a monofilament""s Jc profile or variation. However, these reports only relate to Jc measurement and they never suggests any information useful in processing a multifilamentary wire.
One object of the present invention is to provide a multifilamentary oxide superconducting wire having a relatively high Jc and being suitable for a stranded wire and a method of producing the same.
Another object of the present invention is to provide a low aspect ratio, multifilamentary oxide superconducting wire having a relatively high Jc and a method of producing the same.
Still another object of the present invention is to provide a multifilamentary oxide superconducting wire suitable for a solenoid coil and a method of producing the same.
Still another object of the present invention is to provide a stranded oxide superconducting wire having a relatively high Jc and a method of producing the same.
The present invention is directed to a method of producing a multifilamentary oxide superconducting wire, which includes the step of making a cut or cuts in a tape-shaped multifilamentary oxide superconducting wire prepared by a powder-in-tube process, along the longitudinal direction thereof, so that a plurality of multifilamentary oxide superconducting wires smaller in width than the tape-shaped wire are obtained from the tape-shaped wire.
Furthermore the present invention is directed a method of producing a stranded oxide superconducting wire, which includes the steps of: making a cut or cuts in a tape-shaped multifilamentary oxide superconducting wire prepared by a powder-in-tube process, along the longitudinal direction thereof, so that a plurality of multifilamentary oxide superconducting wires smaller in width than the tape-shaped wire are obtained from the tape-shaped wire; and twining a required number of the multifilamentary oxide superconducting wires obtained.
In the present method, preferably, the tape-shaped wire to be cut has an aspect ratio of at least ten. The obtained multifilamentary oxide superconducting wire smaller in width preferably has an aspect ratio of one to two. In the present method the oxide is typically a bismuth-based oxide.
The present invention is also directed to a multifilamentary oxide superconducting wire. The multifilamentary wire is obtained by the method defined above. The multifilamentary wire includes a stabilizer and a plurality of filaments made of an oxide superconductor in contact with the stabilizer, wherein its aspect ratio is one to two, the plurality of filaments are in the shape of a ribbon extending in the longitudinal direction of the multifilamentary wire, the plurality of filaments are arranged substantially parallel to each other and arranged in layers, the plurality of filaments have an aspect ratio greater than that of the multifilamentary wire, and the plurality of filaments include a filament having a portion substantially not covered with the stabilizer.
In the present multifilamentary wire, preferably, the filaments have an aspect ratio greater than two, more preferably have an aspect ratio of at least ten. In the present multifilamentary wire the oxide superconductor is typically a bismuth-based oxide superconductor.
Furthermore the present invention is directed to a stranded oxide superconducting wire which is composed of a plurality of the multifilamentary oxide superconducting wires defined above that are twined into the stranded wire. Each strand constituting the stranded wire has properties of the multifilamentary oxide superconducting wire defined above.